This invention relates to remoistenable hot melt adhesive compositions having superior non-blocking properties at high temperature and high humidity.
Remoistenable adhesives are commonly used in the production of envelopes, stamps, wallpaper, tapes, labels and similar products wherein it is desirable to provide a dry adhesive film which on being moistened with water will become tacky and thereby allow for the ready adhesion to a chosen substrate. In the manufacture of remoistenable adhesive products, typically an aqueous solution or dispersion, or an organic solvent solution of the dry adhesive is applied as a wet film to a substrate. After the water or organic solvent is removed by drying and evaporation, the resulting dry adhesive film will, on being moistened, produce the desired tacky adhesive surface.
In addition to remoistenability, another desirable property for these applications is non-block. Blocking is an undesirable phenomenon which occurs when the coated substrate, such as the envelopes, are stored in stacks, and exposed to varying degrees of humidity, resulting in the tendency of adjacent surfaces of the stacked substrates to adhere to one another.
Hot melt adhesives are used commercially for a wide variety of applications. The major advantage of hot melt adhesive systems is the lack of a carrier fluid which eliminates the need for drying the adhesive film once it is applied to the substrate. This elimination of the drying step overcomes hazards associated with solvent usage and also allows for faster production line speeds and lower transportation costs.
Hot melt adhesives have historically been based on petroleum derived polymers such as polyethylene, ethylene-vinyl acetate, styrenic block copolymers, polypropylene, and vinyl pyrrolidone polymers including vinyl acetatelvinyl pyrrolidone copolymers. All these adhesive compositions are further tackified, plasticized and/or reinforced with a variety of resins, oils and/or waxes which are derived from both petroleum and naturally occurring feedstocks such as wood, gum and tall oil rosin and terpenes.
Hot melt formulations based on ethylene vinyl acetate base polymers have humidity blocking resistance, but are moisture insensitive, i.e., they do not become tacky when exposed to moisture.
Sulfonated polyesters, when used as the base polymer in hot melt adhesives, become tacky when exposed to moisture. U.S. Pat. No. 5,750,605 discloses that the incorporation of specific crystalline wax materials into hot melt adhesive compositions containing certain sulfonated polyesters has the unexpected advantage of improving both the heat resistance as well as the adhesive strength thereof. Such adhesives find particular use in paper-bonding applications where their water dispersibility facilities repulping of adhesively bonded paper products.
It has been found in accordance with the present invention, that the use of an anionic or cationic plasticizer in conjunction with sulfonated polyesters provides both excellent remoistenability and excellent humidity blocking resistance, while maintaining water dispersibility.